As is well known, thermal imaging systems such as thermal cameras and the like are often implemented with focal plane arrays (FPAs) to detect and capture thermal images from a scene. However, high intensity thermal energy sources (e.g., the sun or other sources) are often problematic for such devices.
In this regard, an intense energy source in the target scene may cause portions of the FPA to become saturated. If the energy source is sufficiently intense, a “burn-in” event may occur wherein affected portions of the FPA remain highly saturated even as subsequent thermal images are captured. This can result in one or more blemishes (e.g., streaks or hot spots) remaining present in the additional thermal images, even after the energy source is removed from the scene.
Existing techniques to mitigate the effects of burn-in events are often unsatisfactory in practice. In one approach, shielding in the form of coatings or filters may be used to protect thermal imaging systems from high intensity energy. However, such shielding often fails to completely eliminate burn-in due to both in-band and/or out-of-band irradiance. In addition, such shielding typically entails a non-trivial cost and may reduce in-band transmission unsatisfactorily.
In another approach, a periodic flat-field correction (FFC) process may be performed in which an FPA is exposed to a uniform scene (e.g., a shutter or other controlled scene), to determine correction terms that generate a uniform output. By applying the FFC terms, the blemishes may be temporarily compensated in subsequent thermal images.
However, as affected portions of the FPA gradually decay to their normal operating states, the magnitude of the offset difference between affected and unaffected portions of the FPA will change over time in the captured thermal images. Moreover, the decay rate is typically non-linear and variable (e.g., ranging from minutes to months depending on detector properties, magnitude of irradiance, exposure time, and other variables). As a result, the blemishes will reappear between periodic FFC processes (e.g., typically as an inverse/negative “dark” blemish) due to overcompensation by the correction terms. Consequently, periodic FFC is also an unsatisfactory mitigation approach.